Not sure why Matthew's solution didn't work for me (could be that I'm using OSX10.8 or perhaps something to do with macports). But I added the following to the end of the file at ~/.profile
export PYTHONPATH=/path/to/dir:$PYTHONPATH
my directory is now on the pythonpath -
my-macbook:~ aidan$ python
Python 2.7.2 (default, Jun 20 2012, 16:23:33)
[GCC 4.2.1 Compatible Apple Clang 4.0 (tags/Apple/clang-418.0.60)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/path/to/dir', ...
and I can import modules from that directory.